1. Field of the Invention
The present invention relates to a method to manufacture composite polymer electrolyte membranes coated with inorganic thin films for fuel cells and applications of these membranes; more particularly to a method of coating the surface of commercial polymer electrolyte membranes with inorganic thin films using a plasma enhanced chemical vapor deposition (PECVD) method or a reactive sputtering method, thus reducing the methanol permeability without a sizable decrease of ionic conductivity, thereby realizing a lower methanol permeability than that of conventional Nafion® membranes or other composite polymer electrolyte membranes and, therefore, enhancing the performance of methanol fuel cells, and also relates to composite polymer electrolyte membranes coated with inorganic thin films for fuel cells, which are manufactured by said method
Also, the present method relates to an membrane-electrode assembly (MEA) employing composite polymer electrolyte membranes coated with inorganic thin films for fuel cells, which are manufactured by the aforementioned method, and a method to manufacture the same.
2. Description of the Related Art
A direct methanol fuel cell (hereinafter referred to as “DMFC”) has the same structure and operates on the same principle as a polymer electrolyte membrane fuel cell (hereinafter referred to as “PEMFC”) using hydrogen, but in case of the DMFC, methanol is directly fed to the anode as a fuel instead of hydrogen. Therefore, its fuel feeding system and overall device is simple compared with the PEMFC, which makes it available in a compact-size. Also, the DMFC has other advantages that the liquid fuel composed of methanol and water functions as a coolant as well as a fuel, the whole device is compact and light-weighted, the operating temperature is much lower than that of the conventional fuel cells, and it can operate for a longer duration at a time due to its convenient refueling.
However, the DMFC has drawbacks that its electrode performance is low due to the methanol oxidation at the cathode side, the platinum catalyst is poisoned by carbon monoxide which is one of reaction products, and the power density is lower than that of PEMFCs. Also, the DMFC has other drawbacks of excessive consumption of expensive platinum catalyst and gradual performance degradation. Yet, the most serious problem of the DMFC is the degradation of its cell performance due to methanol crossover from the anode to the cathode.
The DMFC can overcome limitations on small-sized batteries and inconveniences caused by recharging needs and, therefore, has high prospects of being used as portable power sources for mobile phones, PDAs, and notebook computers. Further, with more improvement in performance, the DMFC could be made available as an automobile power source.
In these DMFCs, an electrolyte membrane carries out not only the role as a proton conductor from the anode to the cathode but also the role as a barrier to methanol and oxygen. Therefore, polymer electrolyte membranes for fuel cells should have a high ionic conductivity and yet a low electronic conductivity. Also, polymer electrolyte membranes for fuel cells should transfer less methanol or water, and be highly stable mechanically, thermally, and chemically.
However, although Nafion® membranes of Du Pont in general use or other commercially available membranes have a superior ionic conductivity, they have the problem that methanol is permeated from the anode to the cathode. This permeated methanol is oxidized on the cathode, poisoning the platinum catalyst thereby causing mixed potentials and, therefore, degrading the whole performance of the cell.
Lots of researches have been performed to resolve this crossover problem in DMFCs. The researches are carried out in two different directions. One is to develop new polymer electrolyte membranes; the other is to improve conventional commercial polymer electrolyte membranes.
As a former example, U.S. Pat. No. 6,503,378 describes a method of manufacturing a composite polymer electrolyte membrane superior in thermal, chemical, and mechanical characters, in which the polymer electrolyte membrane comprised of a hydrophobic hydrocarbon region and a hydrophilic region that are covalently bound to form a single polymer molecule. However, this method is short of reducing the methanol crossover.
Korean Unexamined Patent Publication No. 2002-0004065 describes a method of manufacturing partly fluorinated copolymers based on vinyl compounds substituted with trifluorostyrene, and ionic conductive polymer electrolyte membranes made of the same. It is described that electrolytes can be manufactured with a superior mechanical property at low cost and the swelling can be reduced compared with conventional cases. Yet, it does not report that the methanol permeability can be reduced.
Korean Unexamined Patent Publication No. 2002-0074582 describes a method, in which mixed polymer solutions are made by adding perfluorinate ionomers (eg. Nafion® solution) in polymer matrix, and then polymer membrane is manufactured by casting method, and composite membrane is obtained by coating the perfluorinate ionomers on both sides of the membrane. This method is described to manufacture composite membranes with a superior performance characteristics at a lower cost compared with commercially available Nafion® membranes. Yet, it has drawbacks that the mechanical property of the composite membrane is inferior and the manufacturing process is complicated.
As a second example of modifying Nafion® membranes, some researchers proposed a method producing Nafion®/silicon oxide composite membranes via sol-gel reaction using Nafion® 115 and tetraethylorthosilicate (TEOS) [D. H. Jung, S. Y. Cho, D. H. Peck, D. R. Shin and J. S. Kim, Journal of Power Sources, 106 (2002) 173-177]. This method showed that the methanol permeability decreases with increasing silicon oxide content in the membrane. In cells using this composite membranes according to said method, the current density was 650 mA/cm2 at a cell voltage of 0.5 V and temperature of 120, which is a superior result when compared with other commercial membranes. However, this method has drawbacks that the ionic conductivity is decreased compared with Nafion® membranes and the performance is decreased with increasing silicon oxide content more than 12%.
As another example, some researches proposed a fabrication method in which a polybenzimidazole layer is formed at the surface of Nafion® membrane by screen printing method [L. J. Hobson, Y. Nakano, H. Ozu and S. Hayase, Journal of Power Sources, 104 (2002) 79-84]. The composite polymer electrolyte membrane via this method was shown to reduce the methanol permeability by 40 to 60% and the cell performance was improved by 46%. However, the ionic conductivity has been decreased by about 50% compared with Nafion® membranes.
Also, another method to manufacture membranes has been proposed, which improved the cell performance by 51%. This method performs a surface treatment by exposing the surface of Nafion® membrane in electron beam of 9.2 μC/cm2 at 35kV of accelerated voltage [L. J. Hobson, H. Ozu, M. Yamaguchi, and Hayase, Journal of The Electrochemical Society, 148, 10 A1185-A1190 (2001)]. However, this modified membrane does not reduce the methanol crossover as compared with Nafion® membrane, and shows a drawback that sulfonic groups on the surface are eliminated to a sizable degree.
Therefore, a novel method to manufacture polymer electrolyte membranes for fuel cells is required to improve the fuel cell performance by resolving the drawbacks of conventional polymer electrolyte membranes for fuel cells and even more reducing the methanol crossover.